Major strike-slip faults within the eastern part of the Marlborough fault system of New Zealand extend offshore beneath the continental shelf, into the southern end of the Hikurangi margin. Six major submarine faults, each tens of kilometers in length and exhibiting the three-dimensional structural characteristics typical of strike-slip deformation zones, are mapped in detail using closely spaced high-resolution seismic-reflection profiles. Each fault displaces late Quaternary sediments that have been interpreted within the framework of the sequence stratigraphic model and the established history of glacio-eustatic sea-level cyclicity. Some of the faults are associated with strike-slip microearthquake focal mechanisms.
The faults are within a complex structural high that underlies much of the outer continental shelf, and within the Flaxbourne basin between the structural high and the coast. The plan-view pattern of faulting recorded in the Quaternary sediments reflects only the major surface traces that have propagated upsection as contemporaneous sedimentation has blanketed most of the evolving structures. Regionally, the late Quaternary faults compose two groups that bound blocks with rhomboid surface areas on a scale of tens of square kilometers to hundreds of square kilometers. One group strikes typically between 023° and 057°, i.e., 22°–56° from the plate motion vector (079°), and includes dextral, oblique-slip thrust faults as well as inherited, steeply dipping, pre-Pliocene strike-slip faults that bound thick sedimentary basins. The second group comprises possibly young (<1 Ma), steeply dipping strike-slip faults that commonly exhibit normal-slip separation, and strike between 067° and 085°, subparallel to the azimuth of the instantaneous Pacific-Australian plate motion vector.